selftests/user_events: Fix to unmount tracefs when test created mount

Fix to unmount tracefs if the self-test mounted it to allow testing.
If tracefs was already mounted, this does nothing.

Suggested-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/all/29fce076-746c-4650-8358-b4e0fa215cf7@sirena.org.uk/
Fixes: a06023a8

 ("selftests/user_events: Fix failures when user_events is not installed")

Signed-off-by: Beau Belgrave <beaub@linux.microsoft.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>